1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus which can exchange components of an image forming unit thereof.
2. Description of the Related Art
A conventional electrophotographic image forming apparatus executes image formation using processes described below. Firstly a charging unit uniformly charges a photosensitive drum surface. Then an image processing unit converts image data in an original image (input image signal) based on a gamma look-up table (γLUT) configured to convert input image data stored in the image forming apparatus to thereby make density and gradation characteristics in the original image coincide with density gradation characteristics in an output image. A laser light source uses the converted image data to form an electrostatic latent image on the uniformly charged photosensitive drum surface. The developing unit develops the electrostatic latent image using a toner. The toner image on the photosensitive drum is transferred to a sheet and fixed, and then the image formation is completed. In the case of a color image forming apparatus, the above image forming process is executed for each color. A color image is formed by superimposing toner images for each color on an intermediate transfer member on a sheet during transfer processing.
Image formation characteristics including sensitivity of the photosensitive drum, charging characteristics and development characteristics in the above electrophotographic image forming apparatus are affected by variation in temperature or humidity or accumulate over a number of image forming sheets. Even when the image forming conditions such as charging bias, an exposure amount and developing bias are constant, a toner amount attached to the photosensitive drum may vary with variation in the temperature or humidity or accumulation of the number of image forming sheets. Such a problem occurs because the image forming conditions do not cope with the variation in the image forming characteristics.
To deal with such problem, U.S. Pat. No. 6,418,281 discusses an image density control method for adjusting an output image density by correcting a contrast potential and a γLUT according to variation of the image forming characteristics. The contrast potential means a potential difference between a potential of a developing device and a potential of an exposure unit on the photosensitive drum. A method for controlling image density is configured so that an image forming apparatus forms a patch (reference image) on a sheet to adjust a contrast potential when a user instructs an adjustment for the output image density. A user causes an image reading apparatus provided in the image forming apparatus to read the sheet. The image forming apparatus calculates a patch density using a result read by the image reading apparatus. The contrast potential is set using the difference between the patch density and a target density so that the patch is formed in the target density. Then a gradation test pattern formed from a plurality of patches is formed on the sheet based on the contrast potential set to correct gradation characteristics of the output image. The gradation test pattern is a pattern for generating the above described γLUT which is configured to convert input image data so that the gradation characteristics of the output image coincides with the gradation characteristics of the original image. The γLUT which is configured to convert input image data is generated so that the density of each patch in the gradation test pattern is formed at a target density (to coincide with the gradation characteristics assumed to be ideal for the output characteristics of the image forming apparatus). The input image data is subjected to data conversion according to the generated γLUT and forms as an image based on the converted data.
Since an image can be formed using a contrast potential and the γLUT which correspond to the image forming characteristics at different times by appropriately generating the γLUT or setting the contrast potential, the density and the gradation characteristics of the output image can closely approximate the original image. Image quality can be improved by adapting the γLUT or contrast potential settings to current environmental or apparatus characteristics. Thus, it is desirable to frequently execute control of image density and in particular, to perform image density control after exchanging a photosensitive drum, a charging unit, a developing unit, or the like during maintenance of the image forming apparatus.
However the following problem is associated with an image forming apparatus which can exchange a photosensitive drum, a charging unit, a developing unit, or the like during maintenance of the image forming apparatus. The image density control discussed in U.S. Pat. No. 6,418,281 is executed according to a user instruction and a user must perform an operation associated with reading a sheet on which a patch is formed into an image reading apparatus. As a result, when the user thinks such operation to be troublesome or does not have sufficient time to perform image density control, image formation is executed without γLUT generation or contrast potential settings adapted to the exchanged photosensitive drum, charging unit or developing unit. When the γLUT is not generated or setting for the contrast potential is not performed after exchanging of a photosensitive drum, a charging unit or a developing unit, a conventional image forming apparatus uses the γLUT or the contrast potential which was used for image formation before the component was exchanged. The γLUT or the contrast potential does not correspond to the image formation characteristics after exchange and therefore the problem has arisen that the density of an output image after exchange is different from the density of the original image.
In particular, an image forming apparatus configured to exchange a process cartridge which comprises a photosensitive drum, a charging unit and a developing unit integrated together as one unit and detachably attached to the image forming apparatus main body simultaneous exchanges the photosensitive drum, charging unit and developing unit. There is no guarantee that image density control is surely performed since exchange of the process cartridge of this type of image forming apparatus is executed by a general user rather than a highly skilled service personnel. Consequently, there is the risk that an image forming apparatus configured to exchange a process cartridge may cause a conspicuous deviation between the density and gradation characteristics in the output image and the density and gradation characteristics in the original image.